A wide variety of devices are currently under development utilizing the wider band gap materials such as gallium arsenide or gallium phosphide which provide high intrinsic electron mobility at low fields. Electron mobility of gallium arsenide is between five and six times that of silicon and offers the prospect of greatly increased performance in speed, power or both.
The low field electron mobility of GaAs is one of its greatest attributes and offers high frequency operation in devices such as the field effect transistor (FET). The technology of manufacturing high performance GaAs (FET's) is maturing at a rapid rate and the devices are experiencing a greatly expanding role in oscillator, mixer, logic element, power amplification and low noise/high gain applications especially in satellite communications, military weapon systems, IC test equipment and mainframe computers. However, the full potential has yet to be realized by substrate suppliers and device manufacturers. Present crystal preparation methods do not provide reliable and reproducible high performance devices.
The wider band gap materials, such as gallium arsenide, are all compounds which are subject to complex binary phase equilibria. In every case, one component of the compound, i.e., arsenic, is volatile. Escape of arsenic from the growing crystal causes crystal dislocation defects and vacancies which affect the parameters of the bulk crystal and also affect the structure and performance of epitaxial layers grown on the surface of the crystal.
Precise control of the As vapor pressure in the chosen growth system is required in order to maintain exact stoichiometry of the gallium arsenide compound during the growth process so as to achieve high mobility and crystal perfection.
One process utilized for commercial production of bulk gallium arsenide compound reduces arsenic related defects by providing a high arsenic vapor pressure. A source of arsenic is vaporized and reacts with gallium metal in a sealed quartz ampoule. This is followed by single crystal growth initiated and controlled by establishing thermal gradients for crystal growth across the ampoule (horizontal Bridgman). The resulting semi-circular wafers are small and are not compatible with wafer handling equipment developed for the silicon IC industry. Furthermore, the wafers are of poor quality and still exhibit high inherent defect densities.
The initial thrust of GaAs IC development activities required the growth of an epitaxial GaAs IC layer whose quality was directly dependent upon the quality of the substrate material. The large defect densities inherent in the Bridgman crystal material would directly propagate into the epitaxial layer during its growth phase. The unpredictable and generally poor quality of this material proved a major stumbling block in GaAs IC development.
Mengailis et al (U.S. Pat. No. 3,463,680), Knight et al (U.S. Pat. No. 3, 535,772), Andre (U.S. Pat. No. 3,632,431) and Haurylo et al (3,811,963) all disclose liquid phase epitaxial growth (LPEG) by rotation or tilting an ampoule to flow a melt onto a surface of a III-V substrate to grow an epitaxial layer on that surface. The ampoule is then rotated a second time to move the melt off of the newly formed epitaxial layer.
A much better method of growing crystals is to pull a crystal from a melt according to the Czochralski method which is the standard method for growing most of the device grade silicon. This method cannot be utilized to grow crystals from compounds having volatile components at the crystallization temperature such as gallium arsenide unless the melt is encapsulated with a thin layer of inert liquid such as boric oxide and pressurizing the chamber with an inert gas such as argon or nitrogen. This non-reactive high pressure atmosphere counterbalances the arsenic dissociation pressure. Typical gas pressures during operation can exceed 30 atmospheres and require adequate safety precautions during the use of the crystal pulling equipment. U.S. Patents disclosing liquid encapsulated Czochralski (LEC) growth of crystals are U.S. Pat. Nos. 3,370,927; 3,401,023; 3,472,615; 3,647,389; 3,741,817; 4,303,464; 4,431,476 and 4,478,675.
Larger crystals with lower levels of background impurities are produced than typical Bridgman crystal material. Yields are also significantly higher and this method has proved quite successful and has become commercially feasible.
The temperature gradient at the interface is important since the growing crystal leaves the molten encapsulant environment where radiation heat transfer controls and enters the ambient inert gas where convective heat transfer processes control. Russ in Solid State Technology, August 1972, 29, 31 also reports on the need for temperature control and on the rapid heat loss associated with pulling a crystal from the melt through the thin liquid encapsulant into the gas atmosphere. Russ also reports that the growth process is inhibited if the B.sub.2 O.sub.3 layer is over 2 cm thick. Johnson produced a polycrystalline GaAs by liquid encapsulated floating zone (EFZ) by multiple pass melting of a GaAs rod in a floating zone of B.sub.2 O.sub.3 (J. Crystal Growth 30 [1975] 249-256).
The product of liquid encapsulated growth (LEC) is a single crystal containing substantial stress and stress artifacts (such as dislocations) due to extreme thermal gradients experienced by the crystal during growth and subsequent cool down. The LEC produced crystals are also characterized by inclusions of gallium in the surface of the crystal due to evaporation of arsenic or phosphorous from the surface of the crystal while it is at elevated temperatures when it exits the boric oxide encapsulant.
Crystal defects were reduced by closely controlling the thermal gradients in the growing crystal to reduce the formation of stress-induced defects and also to prevent the loss of one of the constituents of the compound until the crystal has cooled to a point where it is stable. High quality crystals of wide band gap III-VB materials were produced in Ser. No. 706,564 by pulling the crystal from the melt into a temperature controlled column of liquid encapsulant long enough to maintain the crystal submerged until the crystal is relatively cool. Heating and/or cooling elements can be submerged in the column of encapsulant to precisely control and controllably lower the temperature of the liquid in contact with the crystal.
This accomplishes a variety of tasks. One, the volatilization of one component of the crystal is inhibited by encapsulation until the crystal is relatively cool. Second, stresses caused by extreme thermal gradients along the length of the crystal can be reduced by making the environment around the crystal have the temperature gradient most optimum for defect-free growth.
The wide temperature range over which the boric oxide or other encapsulant remains molten makes possible the use of the encapsulant to provide active and passive control of the temperature. The higher heat capacity of a liquid as compared to a gas also provides more sensitive and efficient medium for transmitting or extracting heat from the growing crystal.
The apparatus for continuous production of rod or sheet is fairly large and expensive. It is difficult to maintain precise temperature control along the extended encapsulant path. Furthermore, only a small portion of the III-V and II-VI binary and ternary baths are useful to grow crystals since impurities build up in a binary melt while the composition changes continuously in ternary melts. Since the newly formed crystal is near its melting point, it is a soft material. The pulling forces subject the soft crystal to plastic deformation causing crystal defects.